1. Technical field
The invention relates to measurement techniques, particularly to the field of measurement of the frequency response. For example, included in this field are electrical network and vector-impedance analyzers which measure the transfer coefficient as a function of the frequency. The invention can be used in bio-impedance measurement devices for medical diagnosis, in testers of electrical and electronics circuits, in analyzers of electrochemical elements for their condition monitoring, for investigating of materials by their electrical properties (e.g., conductivity) and also for many other applications.
2. Background art
Solutions are known, where the transfer coefficient of the circuit is measured by applying an excitation signal (e.g., of sinusoidal waveform) to the measured circuit and by multiplying and accumulating of the response signal waveform to this excitation signal, or in other words, by correlating (which is a practical equivalent to mathematical convolution) of these signals, and carried out, in digital implementation, by multiply-and-accumulate (MAC) unit. Also second, quadrature (90 degrees shifted, from excitation signal) reference signal for second correlation (quadrature result component) could be used (U.S. Pat. No. 7,428,683). Correlation calculation can often include also normalization of the result, taking into account average levels and amplitudes (intensities) of the signals.
Such solution is also described in the paper, “FPGA-Based Analog Functional Measurements for Adaptive Control in Mixed-Signal Systems”, JIE QIN et al, August 2007, for BIST, the device consisting of numerically controlled oscillators, digital-to-analog converter and analog-to-digital converter and adjusted for applying excitation signal to an object (e.g., circuit under test and reading back the response signal from the object, and numerical multiplier and accumulator to analyze the properties of the object under test. The disadvantage of such solutions is that the result is calculated by correlation of the reference and response signals as one integral value over the full measurement cycle and therefore such measurement is not showing the transfer coefficient separately for individual frequencies (that means, frequency response) and secondly, such integral measurement is not reflecting correctly and in real-time dynamical, changing in time circuit or object.
For measurement of the frequency response, including dynamic (fast changing in time) circuits and objects, the technical solutions are known, where wideband, e.g., chirp excitation signal are used, and the response signal is analyzed in relatively short sliding time-domain-window by frequency analysis, e.g., by short-time Fourier' transform (STFT) or by wavelet analysis, as described in U.S. Pat. Nos. 6,885,960 and 5,797,840.
The closest solution known in the art is described in the paper “Influence of the analyzing window on electrode impedance measurement by the continuous frequency scanning method”, K. Darowicki, P. Slepski, Journal of Electroanalytical Chemistry, Vol 533, Issues 1-2, 20 Sep. 2002, pp 25-31. In this solution a linear chirp signal is generated for an excitation signal. For dynamical time and frequency domain analysis, a combined time-frequency analysis in the form of short-time Fourier transform (STFT) is used, in which Fast Fourier Transform (FFT) is used in a relatively short-time sliding window, while this window could be weighted by the Gauss window function.
The disadvantage of this solution is the complexity of combined time-frequency analysis of the response signal, as a very sophisticated full spectral Fourier analysis is carried out in every short-time window, demanding a huge processing power and much computing time for calculations. This limits significantly the usage of such solutions for real-time monitoring of the objects and circuits, because and for limited by computational power of processors, which in its turn is limited by cost and available power.
Thus, there is a need for new improved method and device for frequency response measurements